As technology development and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing. Among such secondary batteries, a lithium secondary battery having high energy density and voltage, a long cycle life, and a low self-discharge rate is commercially available and widely used.
However, the lithium secondary battery has a problem in that its service life rapidly decreases as the secondary battery is repeatedly charged and discharged. In particular, this problem becomes more serious at a higher temperature. This is a phenomenon in which an electrolyte is decomposed or an active material is deteriorated due to moisture present in the battery or other influences and the internal resistance of the battery is increased.
Accordingly, a positive electrode active material for the lithium secondary battery, which has been actively researched and developed and is used at present, is a layered LiCoO2. LiCoO2 is most widely used due to excellent life characteristics and charge/discharge efficiency, but LiCoO2 has low structural stability so that there is a limitation in applying LiCoO2to a high capacity battery technology.
As a positive electrode active material substituting for LiCoO2, various lithium transition metal oxides, such as LiNiO2, LiMnO2, LiMn2O4, LiFePO4, and Li (Nix1Coy1Mnz1)O2, have been developed. Among them, LiNiO2 has an advantage of exhibiting battery characteristics of high discharge capacity, but LiNiO2 is difficult to synthesize through a simple solid phase reaction, and has a problem of low thermal stability and low cycle characteristics. Also, lithium manganese-based oxides such as LiMnO2 and LiMn2O4 have advantages of being excellent in thermal stability and being inexpensive. However, the lithium manganese-based oxides have problems of small capacity and poor high-temperature characteristics. Particularly, LiMn2O4 is partially commercialized and used for low priced-goods, but has poor life characteristics due to the structural distortion (Jahn-Teller distortion) caused by Mn3+. Further, LiFePO4 has been studied for a hybrid electric vehicle (HEV) due to its low cost and good safety, but it is difficult to apply LiFePO4 to other due to its low conductivity.
For this reason, a material which is most spotlighted recently as a positive electrode active material substituting for LiCoO2 is a lithium nickel-manganese-cobalt oxide including excessive lithium, that is, Lia1 (Nix2Coy2Mnz2)2-a1O2 (where the a1, x2, y2 and z2 each represent atomic fractions of independent oxide composition elements, and 1<a1≤1.5, 0<x2≤1, 0<y2≤1, 0<z2≤1, 0<x2+y2+z2≤1). This material has advantages of being less expensive than LiCoO2 and being able to be used for high-capacity and high-voltage batteries, but is disadvantageous in that rate capability and life characteristics at a high temperature are poor.
Meanwhile, when impurities exist on the surface of a positive electrode active material during a process of producing an electrode for a lithium secondary battery, the impurities may affect aging in preparation of electrode slurry during the process of producing the lithium secondary battery. In addition, the impurities may react with the electrolytic solution introduced into the lithium secondary battery, thereby causing a swelling phenomenon in the lithium secondary battery since.
To solve such problems, various methods such as a method for coating of the surface of a positive electrode active material or a method for removing impurities from a surface of the positive electrode active material have been suggested, but these problems have not been satisfactorily solved yet.
Accordingly, it is desperately required to develop a positive electrode active material capable of solving the above problems and also improving the performance of the lithium secondary battery.